Extra Class question of the day: Modulation and demodulation

Modulation is the process of adding some kind of information, including voice and digital information, to a carrier signal. The most common types of modulation that we use in amateur radio are amplitude modulation (AM) and frequency modulation (FM). Single-sideband, or SSB, is a form of amplitude modulation.

To frequency modulate a carrier, a transmitter will sometimes us a modulator that varies the phase of the signal. This is sometimes called phase modulation (PM). One way to generate FM phone emissions is to use a reactance modulator on the oscillator. (E7E01) The function of a reactance modulator is to produce PM signals by using an electrically variable inductance or capacitance. (E7E02) An analog phase modulator functions by varying the tuning of an amplifier tank circuit to produce PM signals. (E7E03)

To boost the higher audio frequencies, a pre-emphasis network is often added to an FM transmitter. (E7E05) For compatibility with transmitters using phase modulation, de-emphasis is commonly used in FM communications receivers. (E7E06)

Amplitude modulation and single-sideband signals are produced using mixer circuits. The carrier frequency and the baseband signals are input to the mixer circuit which produces an amplitude modulated output. The term baseband in radio communications refers to the frequency components present in the modulating signal. (E7E07) The principal frequencies that appear at the output of a mixer circuit are the two input frequencies along with their sum and difference frequencies. (E7E08)

When using a mixer, you must take care not to use too high of a signal at the inputs. Spurious mixer products are generated when an excessive amount of signal energy reaches a mixer circuit. (E7E09)

Single sideband is most often used for phone transmission on the HF bands and for weak-signal operation on the VHF and UHF bands. One way a single-sideband phone signal can be generated is by using a balanced modulator followed by a filter.  (E7E04) A balanced modulator is a type of mixer.  The output of a balanced modulator, however, does not contain the carrier frequency, only the two sidebands.

Modern transceivers use digital signal processing to generate SSB signals. The quadrature method describes a common means of generating an SSB signal when using digital signal processing. (E7E13)

At the receiving station, a modulated signal has to be demodulated. Amplitude modulated signals are often demodulated using a diode detector circuit. A diode detector functions by rectification and filtering of RF signals. (E7E10)

For demodulating SSB signals, you want something a little more sophisticated. A product detector is a type of detector that is well suited for demodulating SSB signals. (E7E11) A product detector is actually a frequency mixer. It takes the product of the modulated signal and a local oscillator, hence the name. In an FM receiver, the circuit for detecting FM signals is a frequency discriminator. (E7E12)

Some modern receivers demodulate a signal entirely in software. These receivers are called software-defined receivers. When referring to a software defined receiver, direct conversion means incoming RF is mixed to “baseband” for analog-to-digital conversion and subsequent processing. (E7E14)

Extra Class question of the day: frequency synthesizers

Most modern amateur radio transceivers use digital frequency synthesizers instead of analog oscillators to generate RF signals. On reason for this is that they are much more stable than analog oscillators. The two main types of digital frequency synthesizers are the direct digital synthesizer and the phase-locked loop synthesizer

A direct digital synthesizer is the type of frequency synthesizer circuit that uses a phase accumulator, lookup table, digital to analog converter and a low-pass anti-alias filter. (E7H09) The phase accumulator is a principal component of a direct digital synthesizer (DDS). (E7H12) The information is contained in the lookup table of a direct digital frequency synthesizer is the amplitude values that represent a sine-wave output. (E7H10)

Both the direct digital synthesizer and the phase-locked loop synthesizer have issues with spectral purity. The major spectral impurity components of direct digital synthesizers are spurious signals at discrete frequencies. (E7H11)

For a more detailed explanation of how direct digital synthesizers work, see the electric druid’s  Synth DIY page.

Another type of frequency synthesizer that’s popular are those that use a phase-locked loop. A phase-locked loop circuit is an electronic servo loop consisting of a phase detector, a low-pass filter, a voltage-controlled oscillator, and a stable reference oscillator. (E7H14) 

A phase-locked loop is often used as part of a variable frequency synthesizer for receivers and transmitters because it makes it possible for a VFO to have the same degree of frequency stability as a crystal oscillator. (E7H17) Frequency synthesis, FM demodulation are two functions that can be performed by a phase-locked loop. (E7H15)

An important specification for phase-locked loop circuits is the short-term stability of the reference oscillator. The short-term stability of the reference oscillator is important in the design of a phase locked loop (PLL) frequency synthesizer because any phase variations in the reference oscillator signal will produce phase noise in the synthesizer output. (E7H16) Phase noise is the major spectral impurity components of phase-locked loop synthesizers. (E7H18)

Another important specification is capture range. The capture range of a phase-locked loop circuit is the frequency range over which the circuit can lock. (E7H13)

From my Twitter feed – 5/1/12

RT @mental_floss: Why is “mayday!” an international distress signal? It comes from the French “venez m’aider,” meaning “come help me!”

Finally got “Unleashing the LM386″ on my blog – amazing old audio amp chip http://t.co/Fi38DV97 #hamr #hamradio

Measuring Battery Capacity w/ an Arduino … very cool! http://t.co/HcBys27g #hamr

From trade magazines: GE Transistor Manual, analog circuit design, HF op amp filters

This time, I have two items from EE Times and one from MicroWaves&RF…..Dan


GE Transistor Manual

Master the first 170 pages of the venerable GE Transistor Manual and you'll be a transistor expert.

The GE Transistor Manual. This editorial by Jack Ganssle reminisces about the old GE Transistor Manual. He notes, “It explains transistor theory in a level of detail that my college classes almost a decade later never approached. Read – and understand – the first 170 pages and you’ll be a transistor expert. But no attempt is made to make the subject easy.” One of the comments contains a link that you can use to download your own copy.

Book excerpt: Analog Circuit Design— A Tutorial Guide to Applications and Solutions, Part 1. Based on the Application Notes of Linear Technology, this book covers the fundamentals of linear/analog circuit and system design to guide engineers with their design challenges. It includes a broad range of topics, including power-management tutorials, switching-regulator design, linear-regulator design, data conversion, signal conditioning, and high-frequency/RF design. VERY good stuff.

Fabricating HF Opamp Filters. Until recently, op amp filters have generally been restricted to circuits operating below 1 MHz. Recent advances, though, are enabling op amps to amplify at frequencies up to 1 GHz.This article explains how to use them for lowpass filters to 150 MHz.